Wave Motion and Interference
9
fere. The name has, however, the sanction of long usage, and will therefore be retained. The principle of interference is of
FIG. 8
such fundamental importance that it will be worth while to impress it upon the mind by a few experimental illustrations.
Fig. 8 represents an apparatus devised by Professor Quincke for illustrating interference of sound. An organ
10
Light Waves and Their Uses
pi|x> is sounded near the base of the instrument. Thence the sound waves are conducted through the two vertical tubes, one of which is capable of being lengthened, like a trombone. They then reunite and are conducted by a
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FIG. 9
single tube to a “ manometric capsule,” which impresses the resulting vibrations on a gas jet, the trembling of the jet being rendered visible in a revolving mirror.
When the two branch tubes are of equal length, the waves reach the flame in the same phase, causing it to
FIG. 10
vibrate, as shown by the character of the image in the revolving mirror, Fig. while, if one of the branches be made half a wave1 longer than the* other, the disturbance disappears, and the image appears as shown in Fig. 10.
A very simple and instructive experiment may be made
1 The length required will depend on the tone of the organ pipe. For middle C (256 vibrations per second) the double length required is two feet.